A simulation study of the shape of β′ precipitates in Mg-Y and Mg-Gd alloys

The metastable beta' phase is a key strengthening precipitate phase in a range of Mg-RE (RE: rare-earth elements) based alloys. The morphology of the beta' precipitates changes from a faceted and nearly equiaxed shape in Mg-Y alloys to a truncated lenticular shape in Mg-Gd alloys. In this work, we study effects of interfacial energy and coherency elastic strain energy on the morphology of beta' precipitates in binary Mg-Y and Mg-Gd alloys using a combination of first-principles calculations and phase-field simulations. Without any free-fitting parameters and using the first-principles calculations, CALPHAD databases and experimental characterizations as model inputs (lattice parameters of the beta' phase, elastic constants and chemical free energy of Mg matrix and interfacial energies of the coherent beta'/Mg matrix interfaces), the phase-field simulations predict equilibrium shapes of beta' precipitates of different sizes that agree well with experimental observations. Factors causing the difference in the equilibrium shape of beta'-Mg7Y and beta'-Mg7Gd precipitates are identified, and possible approaches to increase the aspect ratio of the beta' precipitates and thus to enhance the strength of Mg-RE alloys are discussed. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.